The present invention comprises methods of quenching a continuously cast steel product upstream of a reheat furnace that brings the steel to a uniform initial rolling temperature. One purpose served by the invention is to eliminate or reduce the incidence and severity of surface defects in the steel that occur during reduction rolling. There are a number of inventive aspects of the applicant""s methods that collectively may comprise more than one invention, but for convenience, reference will be made to xe2x80x9cthe inventionxe2x80x9d on the understanding that the term covers the collectivity of inventions claimed herein.
In conventional continuous casting mills with direct hot charging, steel in a caster assembly is cast into a continuous strand, and passes through a strand containment apparatus in which the steel surface is cooled and the strand changes direction from the vertical to the horizontal. The strand is then conveyed to a severing apparatus where it is severed into slabs, blooms, billets or other products. The slab or other product then enters a reheat furnace for heating to a uniform temperature suitable for downstream rolling and other processing.
Problems encountered with plate steel product produced by such continuous casting mills include the tendency for areas around one or more surfaces of the steel product to exhibit brittleness, cracking, sponging, and other surface defects (hereinafter collectively referred to as xe2x80x9csurface defectsxe2x80x9d for convenience). Surface defects are especially prevalent after the interim steel product is subjected to downstream rolling or other stresses. Although the causes of such surface defects are not completely understood, it has been observed that surface defects tend to occur frequently in steel products having surfaces that are at or above the steel""s austenite-to-ferrite transformation start temperature (Ar3) when the product exits the caster assembly, and which cool to a temperature above the steel""s austenite-to-ferrite transformation completion temperature (Ar1) as the product enters the reheat furnace, then are reheated to a temperature above the transformation start temperature when the product is inside the reheat furnace. Steel products that tend to be particularly susceptible to surface defects include low- to high-carbon steels and low-alloy steels, all of which may contain aluminum (Al) and residual elements such as sulphur (S), phosphorus (P), nitrogen (N), and copper (Cu).
While an understanding of the causes of the surface defects is not per se necessary for the practice of the invention, some discussion of the applicant""s understanding of the phenomenon may be helpful to the reader. Steel product exiting the caster assembly has a coarse austenite grain structure. As the steel product cools to a temperature above the transformation completion temperature Ar1 of the metal, various elements including residual elements migrate to the austenite grain boundaries where they will reside as solute elements, or eventually combine to form precipitates. If the steel product has not cooled to below the transformation completion temperature Ar1 before reheating in the reheat furnace, these elements, in either solute or precipitate form, remain at or near the partially transformed austenite grain boundaries. The presence of these elements on grain boundaries and/or the development of precipitate-free zones adjacent to grain boundaries can be detrimental to the ductility of the steel product and may also contribute to the manifestation of one or more types of surface defects. It appears that the principal culprit in many cases is the copper and/or aluminum nitride present.
If the interim steel product is taken off-line and left for several hours to cool slowly in still air, the entire product will have completely transformed from coarse-grained austenite to other microconstituents, such as ferrite or pearlite. Reheating this product in a reheat furnace to above the Ac3,(about 900xc2x0 C. for most steels of interest) the critical temperature above which there is austenite, re-transforms the product into fine-grained austenite. It has been found that a product having such a fine-grained austenitic microstructure tends to be free from surface defects. However, such slow cooling requires the product to be taken off-line for an undesirably lengthy period of time, thereby slowing down steel production.
It has been found that instead of re-transforming the entire steel product into fine-grained austenite, it is necessary to re-transform only the surface layers to a suitable depth to achieve a product that is for the most part free of surface defects. However, off-line slow air cooling to achieve a re-transformed layer of sufficient depth requires an undesirably lengthy time.
Previously known methods have been devised in which a slab is taken offline, immersion-quenched in a quench tank, then returned on-line for transfer into the reheat furnace. In such methods, the temperature of the slab surfaces is often reduced below the Ar1, i.e. the steel""s transformation completion temperature, before the slab is reheated in the reheat furnace. It has been found that an immersion-quenched slab tends to exhibit undesirably inconsistent metallurgical properties along its length. This inconsistency appears to be due to the formation of a lengthwise temperature gradient on the slab prior to its immersion; since the slab is cast from a continuous caster, its downstream portions have had more time to cool than its upstream portions.
In another known method, the casting is spray-quenched prior to severing into slabs and prior to entering the reheat furnace. An example of such a method is described in U.S. Pat. No. 5,634,512 (Bombardelli et al.). According to Bombardelli, quenching the strand is accomplished by a quench apparatus that sprays water under pressure through a plurality of sprayer nozzles onto the surfaces of the strand so that the surfaces are rapidly cooled.
A problem associated with Bombardelli""s teaching is that the quench apparatus tends to create a transformed surface layer having an inconsistent depth and microstructure in steel products that, because of casting line speed variations, have developed irregular transverse and longitudinal temperature profiles along their surfaces prior to entering into the quench apparatus. Because the spray intensity in the Bombardelli apparatus cannot be varied amongst nozzles in a group of nozzles directed at a product surface, a product surface having a non-uniform pre-quench temperature profile will have a non-uniform post-quench temperature profile after being sprayed by the Bombardelli quench apparatus, thereby causing inconsistent surface layer properties, including inconsistent microstructures at any given depth of the surface layer.
The invention comprises a method for in-line quenching a steel product. Apparatus suitable for the practice of the method would include, in downstream progression: (1) a caster mould and a strand containment and straightening apparatus, all within a caster assembly; (2) a severing apparatus for severing the steel product from a strand into slabs or other products; and (3) a reheat furnace for reheating the steel product after it has been severed, as well as the apparatus more particularly described and claimed in U.S. application Ser. No. 09/350,319. The steel is normally conveyed from the caster to the reheat furnace on a plurality of spaced conveyor rolls (table rolls).
According to the invention, quenching is effected by applying a plurality of controlled pressurized sprays of cooling fluid (preferably air-mist) to selected portions of one or more surfaces of the steel product exiting the caster, so as to effect in a surface layer of the steel casting a metallurgical change from the initial austenite to desired microconstituents such as ferrite, pearlite, or other transformation products. The quench effects this change to a desired depth of penetration from the surface of the steel prior to the entry of the steel into the reheat furnace. In the reheat furnace, each quenched surface layer is reheated to a temperature above the Ac3 and re-transformed to finer grains of austenite, thereby reducing the occurrence of surface defects on the eventual steel plate product. In practice, the product is also heated above the Tnr to provide a suitable starting temperature for downstream rolling.
According to an aspect of the invention, a method of quenching a surface of an as-cast steel product (e.g., a slab) conveyed along a casting line of a continuous casting steel mill of the type to which reference has been made above comprises, for given input parameters including casting line speed and selected steel product dimensions, the combination of the following two steps:
(a) applying a differentiated spray of cooling fluid, preferably an air mist of air and water, to the as-cast steel product, the spray characteristics of the spray being selected to be sufficient to quench the product such that a surface layer of the product is transformed from an austenitic to a non-austenitic microstructure; and
(b) heating the product to a suitable temperature preferably to a temperature above Ac3 to re-transform the transformed surface layer into fine-grained austenite thereby inhibiting formation of surface defects in the product.
The differentiation of the above-mentioned spray is preferably effected primarily in the transverse sense; to this end, the spray is preferably transversely adjustable. For example, the spray might be adjustable so that a greater volume of cooling fluid is applied to the inner surface areas of the steel strand and a smaller volume to the edge areas. In other words, such arrangement makes it possible to apply a greater cooling effect to a transversely limited portion of the steel product with a relatively higher surface temperature upon entering the quench apparatus, and a lesser cooling effect to a transversely limited portion of the steel product with a relatively lower surface temperature upon entering the quench apparatus.
Temperature sensors may be used to detect the surface temperature of the steel product across its transverse width, which sensors may then provide output signals to a control unit that calculates the varying degree of cooling required across the transverse width of the steel product in order to cool the steel product to a substantially uniform surface temperature, and controls the quench apparatus accordingly. In this way, the operation of the quench apparatus is preferably selected to respond to the pre-quench transverse surface temperature profile of the as-cast steel product such that the uniformity of the post-quench transverse surface temperature profile of the steel product is enhanced.
The spray according to the foregoing method should preferably reduce the surface layer temperature from a temperature of about 1800xc2x0 F. to a temperature of about 1000-1300xc2x0 F., the latter being below transformation completion temperature Ar1.
The spray may be advantageously divided into a plurality of spray groups, the spray characteristics (such as flow rate, flow pressure) of each spray group being separately and independently controllable relative to the spray characteristics of every other spray group. The spray groups may be advantageously arranged in a rectangular matrix generally coincident with a portion of the slab for the time being exposed to the spray.
The steel is conveyed from the caster along the line by the conveyor rolls and passes between the top and bottom arrays of sprays. The flow rate of cooling fluid applied by each spray group is separately controlled. To the extent reasonably possible, the flow rates of the spray groups are adjusted so that all surfaces of the steel will be quenched to the same uniform surface temperature after the steel exits the quench.
The flow rates of cooling fluid applied by the spray groups are differentially selected in a transverse sense (i.e. perpendicular to the casting line direction), because the steel typically experiences non-uniform transverse cooling. In some situations, differential selection of flow rates of other spray groups in a longitudinal sense may also be useful.
The apparatus for providing each spray group may conveniently comprise one or more longitudinally aligned banks of nozzles, each bank comprising a series of nozzles extending parallel to the direction of the casting line. Optionally, other nozzle groups may comprise transversely aligned rows of nozzles extending perpendicular to the direction of the casting line. Preferably one array of nozzles is positioned above the steel and another counterpart array underneath the steel, so that upper and lower surfaces of the steel may be quenched in a balanced, uniform manner.
Each spray group may advantageously comprise at least one dedicated bank. Depending upon the width of the slab, some of the banks may not overlap the slab as it passes by the spray. Advantageously, for a selected transverse width of the steel product, only those banks of sprays directly above and below the product are operated, leaving the other banks idle. This practice tends to facilitate transverse temperature equalization in the steel. Further, as spray applied to the top of the slab may be more effective, for a given quantity of air mist, than spray applied to the undersurface of the slab, it is preferable to control the spray applied to the top surface relative to the spray applied to the undersurface, so that top and bottom surface temperatures are lowered by the spray cooling to substantially the same temperature. This promotes a more uniform surface microstructure overall.
Optionally, the steel product may contain solute nitrogen and titanium may be added as an alloying element to the steel product while it is liquid prior to casting so that the steel product has a weight ratio of titanium to nitrogen of about 3:1. In such case, it is advantageous to cool the steel product to below 750xc2x0 C. to 800xc2x0 C. prior to quenching the steel product in order to enhance the precipitation of TiN, thereby reducing solute nitrogen content in the steel product.
In the present specification, castings severed into slabs will be discussed by way of example, it being understood that the discussion will also apply, mutatis mutandis, to other castings. In slabs, the surface portions nearer the slab""s edges tend to cool more quickly than the inner (central) surface portions; therefore, the edges will be cooler than the central surface portions when the steel reaches the quench sprays. Accordingly, the spray flow rate per surface area provided by the transversely outermost spray groups will be selected to be less than that provided by the spray groups that spray the inner surface portions of the steel, in order to quench all the surface portions to the same post-quench temperature, within engineering limits.
Also, due to anomalies in orderly progress of the steel through the caster or downstream thereof, the steel sometimes cools unequally in a longitudinal direction, so that downstream surface portions are at a different temperature at a given line location than upstream surface portions when they reach the same location. To quench the steel so that all of its surface is quenched to substantially the same temperature and same depth, the spray intensity may be varied with line speed so that each surface portion of the steel is quenched to substantially the same post-quench temperature and to substantially the same depth. Note that such selection or adjustment may be partly space-sensitive and partly time-sensitive; if longitudinally adjustable spray groups are provided, at least some adjustment may be selected by varying the flow rates through such groups or selectively turning selected ones of such groups off or on. If such longitudinally adjustable spray groups are not provided, then longitudinal adjustment of quench spray must be effected by varying over time the flow rates in the available spray groups. Differential longitudinal control of spray is discussed further below.
According to a related apparatus aspect of the invention more particularly described and claimed in applicants"" U.S. patent application Ser. No. 09/350,319, the appropriate selection of flow rate for each spray group is determined by a control unit. The control unit, which may include a general-purpose digital computer or a special-purpose microcontroller, has a plurality of input terminals for receiving data signals from a plurality of input devices, and a plurality of output terminals for controlling a plurality of output devices that collectively serve to control the flow rate and optionally other spray characteristics (e.g., pressure, nozzle spray pattern, if controllable) of each spray group. The input devices may include, for example, a plurality of temperature sensors disposed upstream and downstream of the quench apparatus for measuring the temperature of selected surface portions of the steel entering and exiting the quench apparatus respectively, a casting width setting, and a rotational speed sensor associated with the conveyor rolls for measuring the speed of the steel passing through the quench apparatus.
The control unit processes the data signals received from the speed and temperature sensors and any other input devices, and then, using empirically derived cooling history data for the type of steel being cast, selects the spray groups that will be operable above minimum flow rate, and calculates for each of those selected groups the preferred flow rate, pressure and any other controlled spray characteristics. Then, the control unit sends control signals to the output devices (including, for example, flow rate control valves and pressure regulators downstream of pumps and compressors), so that the flow rate and any other controlled parameters such as spray intensity are set for each group of nozzles. If the quench apparatus is quenching severed strand segments such as slabs, the control unit may also send control signals to one or more conveyor roll drive units to adjust the speed of the rolls, and thus, the speed of the slab passing through the quench apparatus. The foregoing series of operations are continued on a cycling basis by the computer or microprocessor; input values are constantly monitored and as changes occur, output values are modified accordingly.
While a control unit of the foregoing type may advantageously operate mostly or wholly automatically, the system can be designed so that an operator, by using a manual input device communicative with the quench apparatus, may input data or may manually control the quench apparatus. Thus, the operator may operate the quench apparatus under the control of the control unit, or may instead override certain aspects of the control unit""s operation.
Various methods of controlling the rate of discharge of cooling fluid from the various groups of nozzles can be devised. Individual nozzles may be provided with individually controllable valves, or a bank or group of nozzles may be controlled from a single valve. The valve may be a simple off/on valve, or may be an adjustable flow-rate valve, or some combination of the foregoing alternatives may be provided.
One optional transverse flow-control technique proceeds on the premise that the surface temperature profile from one edge of the casting to the longitudinal centre of the casting will gradually increase, and then will gradually drop off to the other edge of the casting; the temperature profile about the longitudinal center line of the casting is generally symmetrical. This symmetry enables flow control valves to be grouped in longitudinally aligned banks, with banks equidistant from the longitudinal center controlled by the same valve. On each side of the longitudinal center line, more than one longitudinal bank of nozzles may be grouped together to form, with its mirror image on the other side of the center line, a single group. In such arrangements, each group of nozzles may be controlled as a unit by means of a single valve, or alternatively the flow rate for any given group may be set to be some constant fraction of the maximum flow rate delivered to the central group of nozzles. (The maximum flow rate would normally be expected to be delivered to the central group because the transverse temperature profile reaches a maximum there.).
It is possible, instead of or in addition to varying the flow rate for a given longitudinal bank of nozzles at any given transverse distance from the center line of the casting, to selectably idle those banks of nozzles that are more remote from the center line, where reduced cooling is required in the vicinity of the transverse extremities of the casting. In the simplest case, given that the entire nozzle array will be designed to accommodate castings of maximum width, the outermost banks of nozzles can be idled whenever the casting being produced is less than maximum width. However, it may be desirable not only to idle those banks of nozzles that are offset outwardly from the transverse edges of the casting, but also those banks that overlap the side edges of the casting. The reason is that the side edges tend to cool more quickly than the central portions of the casting, and also surplus cooling fluid tends to migrate toward the side edges, so idling nozzles that overlap the casting edges may give optimum results.
Note that for all or most banks of nozzles, xe2x80x9cidlingxe2x80x9d involves continuing at least some minimal flow of fluid through the nozzles in order that the nozzles are not damaged by the heat from the casting. In order to save water, and to avoid excessive cooling of the casting, such idling groups of nozzles may be operated on a pulsed basis, so that they pass no fluid for a period of time, and then pass a minimal heat-damage-avoiding amount of fluid for a second period of time, cycling between the two modes.
It may also be desirable to set the flow rate for the nozzles at the input end of the quench unit at a higher level than nozzles downstream, in order to impart a rapid initial surface quench to the steel. This setting, if this option is selected, may be fixed or variable, and would normally be independent of the longitudinal spray control adjustment to compensate for variations in casting speed, discussed next. In certain circumstances and especially with respect to quenching crack-sensitive materials (such as high carbon steels, or high carbon low alloy steels), the flow rate may be set lower at the input end and higher at the output end to avoid initiating the formation of cracks caused by the shock of the quench, or aggravating any cracks that may have formed in the caster assembly 21.
As mentioned, it may be desirable to provide some degree of adjustment of fluid flow from the nozzles in response to changing line speed (i.e., in response to the changing speed of the casting in a longitudinal direction). Such speed changes arise from both normal and anomalous conditions; while complete stops of the casting line are rare except at the end of a casting run, it is not unusual for casting speeds in state-of-the-art steel mills to range from a minimum of about 5 inches per minute to more than 50 inches per minute.
Note that the transversely variable flow control system previously described results in fine control only within the limits available in a configuration in which the nozzles are grouped as selections of longitudinally aligned banks of nozzles. It is contemplated that each longitudinal bank would occupy most of the longitudinal space available to such bank within the quench chamber. The foregoing, therefore, does not take into account the possibility that the designer might wish to regulate flow rate longitudinally on a fine-control basis from the upstream inlet port of the quench unit to the downstream outlet port of the quench unit for the reasons described previously. Such fine control of the quench spray over a longitudinal interval of the casting line is difficult to implement using only longitudinally aligned banks of nozzlesxe2x80x94such groups would have to be split up into sub-groups in a longitudinal series, or in the limiting case, controlling each nozzle by a discrete valve.
An alternative design approach, if such longitudinal variation in nozzle spray is desired, is to establish a second array of nozzles interspersed with the transversely controlled nozzle array, the second array being actuated on a longitudinally adjustable basis instead of a transversely adjustable basis. To this end, for convenience of installation, the second longitudinally adjustable nozzle array could comprise separate longitudinally-spaced rows or banks of transversely aligned nozzles, and could be provided with supply pipes for the nozzles that extend vertically a greater distance than the supply pipes for the transversely adjustable nozzles, thereby facilitating the provision of different sets of horizontally oriented supply conduits for the transversely variable nozzle array from those for the longitudinally variable nozzle array, the two sets of supply conduits being perpendicular to one another. An individually adjustable valve could be provided for each such transversely extending bank of nozzles; again variable control or simple on/off control for each such bank could be provided. If some transverse temperature profile is desired for the spray to be applied by the longitudinally variable nozzle arrays, yet fine control is sought to be avoided as unduly complex or expensive, the nozzle size could vary over the transverse span of each row of such nozzles, with the nozzles overlying the central inner areas of the surface of the steel providing more flow of cooling fluid than those nozzles overlying the outer surface areas of the steel.
In considering the effect of changing casting speed upon the quench arrangement, account must be taken of the fact that problems arising from abrupt cooling of the casting caused by sudden deceleration of the casting line speed usually require a more rapid response than problems associated with casting line speed increase. Accordingly, the flow rate of fluid through the nozzles should decrease appreciably if there is a significant deceleration in casting line speed. By contrast, acceleration of casting line speed may require a more modest response by the flow-control system; an increase in flow rate of less than half the decrease associated with a line speed deceleration may be adequate. Severe over-quenching tends to be more of a potential problem than under-quenching; temperature feedback control from a pyrometer or other temperature monitoring device upstream and downstream of the quench facilitates avoidance of over-quenching. Severe over-quenching can cause severe distortions in the steel, and even cracking or breaking of some grades of steel. Such over-quenching is of particular concern with crack-sensitive materials.
Note that because of the need to provide at least some minimum rate of flow through the nozzles to prevent damage to the nozzles, fine control over quench flow-rates for very slow-moving castings may be difficult or impossible to achieve. In practice, this tends not to present a problem for mild over-quenching of the castingxe2x80x94mild over-quenching has the negative consequence that more heat is required in the reheat furnace to bring the casting up to the initial rolling temperature, but otherwise there is no significant, if any, metallurgical damage to the surface of the casting by quenching to a somewhat deeper layer than is considered optimal. Nevertheless, severe over-quenching is to be avoided for the reasons already mentioned.
The choices of nozzle banks to be controlled together, of nozzle spacing and sizing and maximum flow rate, of minimum flow rate and whether idling nozzles should be pulsed or run continuously at minimum flow rate, of flow rate for specified casting speeds, of the nozzle banks chosen to be active for a casting of a specified width, of the acceleration and deceleration of flow rate in response to acceleration and deceleration of casting line speed, and similar such design choices, may be made empirically on the basis of trial runs. If surface cracks are not occurring in the finished product, the choices made will generally prove to have been sound from a metallurgical standpoint. It remains to provide for reasons of economy the minimum quenching compatible with a good metallurgical result, because too much quenching costs money; more heat is required in the reheat furnace to bring an over-quenched casting up to uniform target pre-rolling temperature.
For a given nozzle array, the designer has to select the number of nozzles to be provided for the quench apparatus, their spacing from one another, the number of banks of nozzles to be under the control of a single valve (or operating in response to a single control signal), maximum and minimum flow rates per nozzle, the ratio of casting speed to nozzle flow rate in a given active bank, the ratio of flow rates in the outer banks of nozzles relative to the flow rates provided for the central bank, etc. For optimal results, any such design should be tested on an empirical basis.
Whether a steel product has been satisfactorily quenched is typically determined empirically; to this end, a quenched test portion of the steel may be removed from the line downstream of the reheat furnace. The cross-section of the test portion is then examined to determine whether the flow provided by each spray group has been appropriately selected or adjusted by the control unit. For a given slab, the steel layers adjacent to the top and bottom surfaces are examined to determine whether the quench has suitably transformed the steel""s microstructure, and whether the depth of transformation is satisfactory. A series of such measurements and observations can be used to calibrate the control unit and the operating mechanisms that adjust selected controlled spray parameters.
Occasionally there is a line interruption of sufficient duration that the quench should be discontinued. In such situations, the use of the present invention may be insufficient to prevent surface defects; the steel may have to be downgraded or conceivably even scrapped. In such cases, the flow through the spray nozzles is reduced but not completely interrupted, so that the continuous flow of fluid through the nozzles cools the nozzles sufficiently to prevent damage to the nozzles. Note that below some minimum flow rate per nozzle, the nozzle spray pattern may become restricted or irregular, causing non-uniformity of surface quench. The system should be designed to avoid normal operation below such minimum flow rate.